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Title: Optical and spectroscopic analysis of phase transitions in the bulk and aerosols of liquid hydrocarbon fuels and their ethanol blends
Author: Corsetti, Stella
ISNI:       0000 0004 5992 7118
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2016
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This thesis reports the investigation of liquid-to-gas and liquid-to-solid phase transitions of liquid hydrocarbon fuels and their ethanol blends, both in the bulk phase and as single droplets. The key point has been to develop an understanding of the fuels' macroscopic behavior by studying them at the molecular and at the single droplet level. A key work in this thesis is the investigation of different ratio ethanol/gasoline blends at the molecular level. At the macroscopic level, the vapor pressure, and hence the evaporation of the blends, is influenced by the strength of intermolecular interactions. Thus, information on the molecular interactions between ethanol and gasoline are inferred by using IR and excess IR spectroscopy. The spectroscopic data suggest that the hydrogen bonding between ethanol molecules is weakened upon gasoline addition, but the hydrogen bonds do not disappear. This can be explained by a formation of small ethanol clusters that interact via Van der Waals forces with the surrounding gasoline molecules. In addition, Raman spectroscopy is performed on the same blends, and the Raman spectra are compared with the IR ones. Two different approaches for data evaluation, with the scope of determining the ethanol content in the blends, are tested and compared: Firstly, the calibration of the intensity ratio of characteristic peaks as function of composition; secondly, a principal component regression (PCR). Both methods are found to have comparable uncertainty. For the evaluation of the Raman spectra, the PCR method yielded better accuracy than the intensity ratio approach. In addition, a detailed investigation of the influence of noise in the signal is presented. When the full IR spectra were evaluated by PCR, even high noise levels did not reduce the measurement accuracy significantly. Later, with the aim of studying the evaporation dynamics of fuel blends, at the single droplet level, electrodynamic balance (EDB) and optical tweezers are used to trap ethanol/gasoline droplets, containing different ethanol percentages. A longer lifetime is observed for droplets containing a greater fraction of ethanol. In order to explain the experimental evaporation trends obtained, a theoretical model is used to predict the evaporation rates of pure ethanol and pure gasoline droplets in dry nitrogen gas. Also a theoretical estimation of the saturation of the environment, with other aerosols, in the tweezers is carried out. Lastly, the liquid-to-solid phase transition of some long chain alkanes, commonly present in diesel or gasoline, is investigated both at the molecular and at the single droplet level. Firstly, by using Raman spectroscopy the solidification of these hydrocarbons in the bulk phase is observed. Distinctive features associating the solid even hydrocarbons to a triclinic structure and the odd ones to an orthorhombic structure can be observed in the spectra. Secondly, the liquid-to-solid phase transition of single hydrocarbons droplets is investigated. Freezing time and surface area resulted to be inversely proportional in dodecane droplets. This might suggest a surface freezing mechanism. Furthermore, differences in the scattering patterns, depending on the freezing mechanism, are pointed out. Droplets freezing homogeneously show a different scattering pattern with respect to droplets that froze heterogeneously.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Hydrocarbons ; Liquid fuels